TOKYO/WASHINGTON Japan's military is considering joining the United States in maritime air patrols in the South China Sea in response to China's increasingly assertive pursuit of territorial claims, a Japanese and a U.S. source familiar with the discussions said.

News of the deliberations came as Japan and the United States unveiled new defense guidelines during a visit to Washington by Japanese Prime Minster Shinzo Abe, reflecting his plans for Japan to take on a wider security role beyond the direct defense of its home islands.

While no concrete plans had yet been formulated, Japan could join U.S. patrols in the South China Sea, or operate patrols in rotation from the Japanese island of Okinawa on the edge of the East China Sea, the Japanese source said, speaking on condition of anonymity.

So far the discussion is within the Japanese military, but any move to begin patrols would need civilian approval.

Japanese air patrols in an area where China is pushing disputed territorial claims, including though a massive program of land reclamation, would risk antagonizing Beijing.

But defense officials in Tokyo worry that doing nothing would allow China to eventually impose its authority over a region through which $5 trillion of sea-borne trade passes ever year - much of it heading to and from Japan.

"We have to show China that it doesn't own the sea," said the Japanese source.

FREEDOM OF NAVIGATION

A U.S. military source said a decision to begin flights in the South China Sea could prompt Tokyo to ask the Philippines for access to air bases under disaster relief training and other joint training exercises. This would give Japanese aircraft added range to stay out on patrol longer, he said, also speaking on condition of anonymity.

A senior Philippine military source said such access would not currently be possible, because Manila does not have any military co-operation agreement with Tokyo similar to the pact it has with Washington, which allows U.S. ships to use its bases to re-fuel, re-supply and make emergency repairs.

But Philippine President Benigno Aquino, one of the most vocal regional critics of China's reclamation program, is due to meet Abe in Tokyo in June, when the South China Sea issue is certain to be discussed.

U.S. President Barack Obama told reporters after talks with Abe on Tuesday that the two countries shared concerns about China's reclamation and construction activities in the South China Sea and were "united in our commitment to freedom of navigation and respect for international law".

He said the new defense guidelines meant U.S. and Japanese forces would be more flexible, and that Japan would "take on greater roles and responsibilities in the Asia-Pacific".

Neither he nor Abe detailed what those roles might be.

Japanese Defence Minister Gen Nakatani and Foreign Minister Fumio Kishida dodged questions about the possibility of joint patrolling of Asian sea lanes when pressed at a news conference with U.S. counterparts on Monday, saying legislation in Japan had still to be worked out and regional countries consulted.

Speaking at a daily briefing in Beijing on Wednesday, Chinese Foreign Ministry spokesman Hong Lei said the United States and Japan were "not involved in the South China Sea issue" and should not do anything to "complicate the situation".

REGIONAL COUNTERWEIGHT

The United States has a squadron of Lockheed Martin (LMT.N) P-3 Orion patrol planes and a squadron of new Boeing (BA.N) P-8 Poseidon sub-hunting aircraft in the region, six of which are stationed in Okinawa.

Japan operates a fleet of 70 P-3s in the seas around Japan and is due to deploy about 20 new Kawasaki Heavy (7012.T) P-1 patrol aircraft with twice the range of the older Orions over the next five years.

In an interview in January, Admiral Robert Thomas, commander of the U.S. Seventh Fleet, told Reuters that Washington would welcome Japanese air patrols in the South China Sea, because their presence would provide a stabilizing counterweight to a growing fleet of Chinese fishing and naval vessels.

Thomas's comment, which was criticized by the Chinese government, was quietly welcomed by some Japanese defense officials. It helped "break down a psychological barrier" that had made discussion about possible operations in the South China Sea a taboo, a Japanese defense ministry source told Reuters.

A spokesperson for the U.S. State Department said it was "not aware of any official plans or proposals for Japan to patrol the South China Sea" but welcomed "a more active role for Japan in ensuring stability and security in East Asia and globally, including in addressing maritime security challenges".

A senior U.S. defense official said last week details of future activities would have to be worked out as Japan completed domestic legislation needed to expand the role of its defense forces as it reinterprets the limits of its pacifist post-war constitution.

China claims about 90 percent of the 3.5 million sq km (1.35 million sq mile) South China Sea. The Philippines, Taiwan, Malaysia, Brunei and Vietnam also claim large parts.

At a regional summit in Malaysia on Monday the Association of Southeast Asian Nations (ASEAN) released their most critical statement yet of China's building of man-made islands on disputed reefs, which Washington and Tokyo worry will become military bases to police the area.

ASEAN said that China's reclamation program "eroded trust and confidence and may undermine peace, security and stability".

(Additional reporting by Manuel Mogato in Manila and Ben Blanchard in Beijing; Editing by Alex Richardson)

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